1. Field of the Invention
This invention relates to a mobile communication field, and more particularly to a broadband dual-polarized omni-directional antenna (BDOA) and a feeding method using the same.
2. Description of the Prior Art
In the Advanced International Mobile Telecommunication (IMT-Advanced) system, both the Time Division Duplexing (TDD) and the Frequency Division Duplexing (FDD) use the Multi-Input-Multi-Output (MIMO) antenna technology.
According to the MIMO antenna technology, multiple transmitting antennas and multiple receiving antennas are used at the transmitting side and the receiving side respectively. The radio signal is transmitted and received through the multiple antennas at the transmitting side and the receiving side so as to improve the quality of service (such as bit error rate or data speed) for each user. For the traditional Single-Input-Single-Output (SISO) antenna system, MOMO antenna system may improve the frequency spectrum utilizing rate and make it possible to provide higher speed data services with limited radio frequency bands.
According to the practical requirement for the MIMO antenna system, the MIMO antenna array may take the form of 2×2 or 4×4 and the mono-polarized antenna or dual-polarized antenna may be used as the array unit for the MIMO antenna system. The mono-polarized antenna refers to the one with dipoles arranging into one column in the same direction and receiving the radio signal from one direction. The mono-polarized antenna may be vertically polarized, horizontally polarized or ±45° polarized with respect to the datum level. The dual-polarized antenna refers to the one with dipoles arranging into two columns in two directions and receiving the radio signals from two directions. Two inner dipoles of the dual-polarized antenna may be polarized differently. For example, one dipole may be horizontally polarized (horizontally polarized antenna) and the other dipole may be vertically polarized (vertically polarized antenna); or one dipole may be ±45° polarized (±45° antenna) and the other dipole may be −45° polarized (−45° antenna).
According to the directionality of the antenna(s), there may be the omni-directional antenna and the directional antenna. The omni-directional antenna refers to the one without the maximum direction when transmitting and receiving radio signal in the horizontal plane, which has a comparatively low antenna gain and a comparatively short transmission distance of the radio signal. Therefore, the omni-directional antenna is mainly adapted to use in a point to multi-point environment which does not have a strict requirement on transmission distance, for example in the indoor environment. Compared with the omni-directional antenna, the directional antenna has the advantages of good directionality, concentration of energy in a specific direction, high gain, comparatively long transmission distance, comparatively strong anti-interference ability and is more adapted to use in a long distance point-to-point communication. The disadvantages of the directional antenna are small coverage, difficult in mounting and adjusting, and requiring the antennas at the two transmission points to be aligned so as to guarantee the transmission of the radio signal.
The current MIMO antenna system is mainly designed for outdoor environment. For the indoor environment, the MIMO antenna system is designed to generally include a plurality of mono-polarized omni-directional antennas that are vertically polarized because of the complexity of the environment and the requirement of a comparatively broadband coverage.
In the current indoor MIMO antenna system that adopts the mono-polarized omni-directional antenna as the array unit, since the frequency utilization rate of the mono-polarized antenna is low, the data transmission rate is comparatively low. In addition, in order to guarantee the high capacity of the indoor MIMO antenna system, the number of the mono-polarized antennas is required to be comparatively large, which leads to a comparatively large occupation of space by the indoor MIMO antenna system. Therefore, an omni-directional antenna that may improve the frequency utilization rate and occupy a comparatively small space is needed to work as the array unit of the indoor MIMO antenna system.
Considering that the dual-polarized omni-directional antenna may separate overlapped frequencies compared with the mono-polarized omni-directional antenna and improve the frequency utilization rate, it is proposed to use the dual-polarized omni-directional antenna as the array unit for the indoor MIMO antenna system, which may improve the frequency utilization rate while occupying a comparatively small space. As shown in FIG. 1, which is a schematic diagram showing a structure of a dual-polarized omni-directional antenna, the dual-polarized omni-directional antenna has one vertically polarized antenna 1 and four horizontally polarized antennas 2. The working frequency of the antenna ranges from 225 MHz to 400 MHz. Therefore, compared with the mono-polarized omni-directional antenna, although the dual-polarized omni-directional antenna may improve the frequency utilization rate and the MIMO antenna system constituted by the dual-polarized omni-directional antenna occupies a reduced space, the dual-polarized omni-directional antenna can hardly be used in the mobile communication system because of the dual-polarized omni-directional antenna working at a comparatively low frequency and a comparative ovality of the horizontal polarization.
Besides the above problems of the MIMO antenna, in the 3rd generation (3G, i.e. TD-SCDMA, WCDMA and CDMA2000) mobile communication system, the indoor antenna still uses the mono-polarized antenna as the 2nd generation (2G) mobile communication system. However, since the 3G system has a comparatively high frequency, the coverage distance of the mono-polarized antenna using the single channel mode is greatly reduced and the number of the antennas has to be multiplied to compensate for the reduction of the coverage distance. The dual-polarized omni-directional (ceiling) antenna using the dual channel mode may meet the coverage requirement of the network with the same number of antennas as that of the 2G network by utilizing the polarization gain effect of the polarization diversity. However, the current dual-polarized omni-directional (ceiling) antenna still works at a comparatively narrow frequency band.
Therefore, a dual-polarized omni-directional antenna that works at a comparatively broad frequency band is needed, which may improve the frequency utilization rate while guaranteeing the frequency band coverage rate so that the dual-polarized omni-directional antenna may improve the data transmission rate while occupying a comparatively small space.